linux-kernel - Re: [PATCH][next] KVM: x86: Fix allocation sizeof argument

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Date:   Wed, 6 Oct 2021 09:22:28 +0900
From:   David Stevens <stevensd@...omium.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Colin King <colin.king@...onical.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        kvm@...r.kernel.org, kernel-janitors@...r.kernel.org,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH][next] KVM: x86: Fix allocation sizeof argument

On Wed, Oct 6, 2021 at 12:41 AM Sean Christopherson <seanjc@...gle.com> wrote:
>
> On Fri, Oct 01, 2021, Colin King wrote:
> > From: Colin Ian King <colin.king@...onical.com>
> >
> > The allocation for *gfn_track should be for a slot->npages lot of
> > short integers, however the current allocation is using sizeof(*gfn_track)
> > and that is the size of a pointer, which is too large. Fix this by
> > using sizeof(**gfn_track) instead.
> >
> > Addresses-Coverity: ("Wrong sizeof argument")
> > Fixes: 35b330bba6a7 ("KVM: x86: only allocate gfn_track when necessary")
> > Signed-off-by: Colin Ian King <colin.king@...onical.com>
> > ---
> >  arch/x86/kvm/mmu/page_track.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/mmu/page_track.c b/arch/x86/kvm/mmu/page_track.c
> > index bb5d60bd4dbf..5b785a5f7dc9 100644
> > --- a/arch/x86/kvm/mmu/page_track.c
> > +++ b/arch/x86/kvm/mmu/page_track.c
> > @@ -92,7 +92,7 @@ int kvm_page_track_enable_mmu_write_tracking(struct kvm *kvm)
> >               slots = __kvm_memslots(kvm, i);
> >               kvm_for_each_memslot(slot, slots) {
> >                       gfn_track = slot->arch.gfn_track + KVM_PAGE_TRACK_WRITE;
> > -                     *gfn_track = kvcalloc(slot->npages, sizeof(*gfn_track),
> > +                     *gfn_track = kvcalloc(slot->npages, sizeof(**gfn_track),
> >                                             GFP_KERNEL_ACCOUNT);
>
> Eww (not your patch, the original code).  IMO the double indirection is completely
> unnecessary, e.g. I find this far easier to follow
>
> diff --git a/arch/x86/kvm/mmu/page_track.c b/arch/x86/kvm/mmu/page_track.c
> index bb5d60bd4dbf..8cae41b831dd 100644
> --- a/arch/x86/kvm/mmu/page_track.c
> +++ b/arch/x86/kvm/mmu/page_track.c
> @@ -75,7 +75,7 @@ int kvm_page_track_enable_mmu_write_tracking(struct kvm *kvm)
>  {
>         struct kvm_memslots *slots;
>         struct kvm_memory_slot *slot;
> -       unsigned short **gfn_track;
> +       unsigned short *gfn_track;
>         int i;
>
>         if (write_tracking_enabled(kvm))
> @@ -91,13 +91,13 @@ int kvm_page_track_enable_mmu_write_tracking(struct kvm *kvm)
>         for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
>                 slots = __kvm_memslots(kvm, i);
>                 kvm_for_each_memslot(slot, slots) {
> -                       gfn_track = slot->arch.gfn_track + KVM_PAGE_TRACK_WRITE;
> -                       *gfn_track = kvcalloc(slot->npages, sizeof(*gfn_track),
> -                                             GFP_KERNEL_ACCOUNT);
> -                       if (*gfn_track == NULL) {
> +                       gfn_track = kvcalloc(slot->npages, sizeof(*gfn_track),
> +                                            GFP_KERNEL_ACCOUNT);
> +                       if (gfn_track == NULL) {
>                                 mutex_unlock(&kvm->slots_arch_lock);
>                                 return -ENOMEM;
>                         }
> +                       slot->arch.gfn_track[KVM_PAGE_TRACK_WRITE] = gfn_track;
>                 }
>         }
>
>
>
> >                       if (*gfn_track == NULL) {
> >                               mutex_unlock(&kvm->slots_arch_lock);
>
> Hrm, this fails to free the gfn_track allocations for previous memslots.  The
> on-demand rmaps code has the exact same bug (it frees rmaps for previous lpages
> in the _current_ slot, but does not free previous slots).
>
> And having two separate flows (and flags) for rmaps vs. gfn_track is pointless,
> and means we have to maintain two near-identical copies of non-obvious code.

I agree that's better than my patch. I can put together a new patch
once it's decided whether or not my patch should be dropped.

-David